The main results of our work are that patients with ACR had significantly lower values of LV-GLS, RV-FWLS and LV-twist and higher level of troponin I than patients without significant ACR and that the combination of troponin and these parameters may improve the diagnostic value of strain in heart transplant patients with preserved left and right ventricular systolic functions. The combination of RV-FWLS and troponin presented a higher accuracy for the detection of ACR degree ≥ 2R. The AUC of the combination of troponin (> 0.05 ng/mL) with RV-FWLS (with a cut-off value < 18%) was 0.89 (95% CI: 0.81–0.93). Thus, it may be considered for use as a screening tool for the detection of ACR and could reduce the number of EMBs after heart transplantation.
ACR is a significant and frequent complication of heart transplantation. In the first year, it is the most common cause of mortality. Currently, EMB is the clinical gold standard in screening for graft rejection after heart transplantation and is actually the only tool for the diagnosis and classification of allograft rejection (5). Considerable efforts have been made to improve the consistency, reliability and reproducibility of the histopathological evaluation of EMB. However, several issues make EMB assessment more difficult and less reproducible than it should be. Critical issues include the subjective and challenging pathological interpretation of EMBs and the risks associated with the procedure (25). Considering these limitations, noninvasive techniques to detect cardiac rejection have been evaluated (14, 26).
To the best of our knowledge, this is the first study to analyze all strain parameters using STE (LV-GLS, LV-GRS, LV-GCS, LV-twist and RV-FWLS) in the same population of heart transplanted patients with normal right and left ventricular systolic function, and its combination with biomarkers, to diagnosis clinically significant ACR. Extra care was given in our study to restrict patient selection to a fixed period of time (6 months post-heart transplantation) to minimize the possible influence of the time since heart transplantation on ventricular strain, as recommended by the current guidelines (27). Additionally, this fixed period of selection minimized the possible bias of pretransplantation ischemic injury, which can manifest up to the sixth month post heart transplantation. The first six months is a period of adaptation, during which many patients can still present some degree of right ventricular systolic dysfunction.
Previous studies have shown that myocardial strain has a higher sensitivity than conventional echocardiography, and therefore, may be an important tool to detect early subclinical cardiac dysfunction (28). Although myocardial strain imaging has been reported to have potential for the detection of graft dysfunction in the early stage, its diagnostic value has not been widely recognized yet (19, 20). Similarly, as reported in a recent meta-analysis, our study showed that the LV-GLS was lower in transplant patients with significant ACR compared with patients without significant ACR (29). This may occur secondary to myocardial deformation, and it may be impaired due to inflammatory cellular infiltration and myocardial edema, and can be reflected by myocardial strain parameters. However, the diagnostic value of other strain parameters by 2D STE on ACR detection is still controversial.
Right ventricular strain analysis has been poorly described to date in this scenario. Mingo-Santos et al. (30). demonstrated a predictive role of STE parameters in the diagnosis of ACR (RV-FWLS and LV-GLS, with threshold values of < 17% and < 15.5%, respectively). That study classified biopsies into 3 groups (0, 1R, and ≥ 2R). However, our study divided the biopsy results into two groups according to the grade of rejection: biopsies without significant rejection (0 and 1R) and biopsies with significant rejection (≥ 2R). This division was based upon the clinical meaning of the rejection grade, since cases with grades of 0 or 1R do not require an immediate intervention via adjustment of immunosuppressive medications, whereas this adjustment is necessary in patients presenting with 2R and 3R rejection. In agreement with the study from Mingo-Santos et al. (30), our study confirms the reduction in left ventricular and right ventricular STE parameters during ACR ≥ 2. Our cutoff value was slightly higher than that reported by Mingo-Santos et al. (30). We speculate that it might be due to the use of different echocardiographic equipment or even the characteristics of the studied populations. Unfortunately, the investigators did not analyze LV-Twist and troponin.
As we showed in our results, the LV-twist values were significantly lower in the group with significant ACR than in the group without significant ACR (13.9° ± 4.79° vs 17.1° ± 3.02°, p < 0.048). In parallel to our results, a unique previous study that applied STE derived LV-twist measurements to detect rejection in heart transplanted patients demonstrated that the LV-twist decreased more in the group with ACR than in the group without ACR (9.6 ± 2.7 vs 12.2 ± 2.3), p < 0.0001) (31). We postulated that twist preced the deterioration in left ventricular ejection fraction, suggesting early myocardial involvement in cardiac rejection. With the advances of technology that have made this technique more available and increasingly feasible, this parameter of cardiac mechanics has been increasingly studied in other pathological situations, and can be applied in this type of patient (32, 33).
As acute rejection promotes cardiomyocyte necrosis and results in compromised cardiac mechanics, troponin and BNP have been evaluated as potential diagnostic tools for ACR (34, 35). There have been controversial results on these biomarkers in the field of heart transplantation (36, 37). Our study used an ultrasensitive assay for cardiac troponin I that detects 10 to 100 times lower levels than standard assays. Troponin was measured before the biopsy, so this procedure did not interfere with its serum levels. Troponin I levels were significantly elevated in patients with significant ACR. Serum BNP levels were not different between the groups; this finding can be explained by the suggestion of some studies that BNP remains altered in most patients for up to one year after heart transplantation (38, 39). In accordance with Bader et al. (40), we also observed that BNP levels did not predict rejection after heart transplantation, and we suggest that BNP is not clinically useful for the detection of acute cellular rejection.
In accordance with previous reports in the literature (41), we confirmed in our population that heart transplant patients have a characteristic cardiac geometric remodeling, featured by a greater septum and posterior wall thickness, a larger left atrium diameter, and greater left ventricular mass index values than matched non transplanted controls. Heart transplant patients also showed lower values for conventional right ventricular systolic function parameters, such as fractional area change, tricuspid annulus systolic velocity and tricuspid annular plane systolic excursion. Moreover, regarding left ventricular diastolic function, heart transplant patients had lower tissue Doppler velocities and higher E/e´ ratios, suggesting impaired relaxation and increased left ventricular filling pressures.
This study demonstrated that heart transplant patients without rejection present unique ventricular dynamics, characterized by lower LV-GLS, LV-GCS, LV-GRS and RV-FWLS, in comparison with control individuals. We have confirmed the data recently published by Ingvarsson et al. (41), which showed that echocardiographic measurements from 124 heart transplant patients were different from the reference values except for LV-CGS. Unfortunately, the investigators did not analyze LV-Twist. In our study, we used a non-transplanted control group matched by age and sex to confirm these results. Multiple mechanisms may explain the different echocardiographic findings in heart transplant patients. Their pathophysiology involves the consequences of surgical trauma, such as ischemic injury and the release of inflammatory mediators, in addition to previous pulmonary hypertension compromising right ventricular dynamics and the risks associated with rejection, cardiac biopsies and immunosuppressive medications.
Our data failed to find any association between diastolic markers and rejection. The results found in the literature are highly conflicting and could not be reproduced by our data. This can be explained by a limitation of diastolic dysfunction parameters due to their dependence on heart rate (which is generally elevated in transplanted patients, with a fusion of E and A waves), loading conditions and donor age (42–46).